Creating renders from Image Stacking using different sampling levels
Short answer:

My guess is that it should be 50%. The combined samples of the images above it is ~750, which is the same as the number of samples in the rendered image. Since the rendered image and the combined images are more or less equivalent, They should be combined with a 50/50 mix.

Long answer:

Each image's opacity is based on how many samples it has compared to the image above it. 

E.g. if you render 5 images with 10 samples each, the second image should be mixed with the first with a factor of .5 (or an alpha of 50%). This will effectively result in a 20 sample image (as you pointed out, it's not quite the same as 20 rendered samples, but it's reasonably close), so the second image should be mixed with a factor of .33 (alpha of 33%) and so on.

Since the resulting combined image is basically a 750 sample render, it's the same number of samples as your 750 sample render. You can just tack it on top with a transparency of 50%.